1. Field of the Invention
The present invention relates to a method for releasing an unmanned missile from a carrier aircraft. In particular the invention relates to a method for releasing an unmanned, aerodynamically unstable missile from a carrier aircraft.
2. Discussion of Background Information
Although conventional unmanned missiles, for example so-called “cruise missiles,” are provided with their own wings as uplift aids, these wings in carried flight, i.e., when the missile is attached to the carrier aircraft, are in a retracted position within the contour of the missile, and are not extended until after the missile is released. The missile is thus in an aerodynamically unstable state directly after the release from the carrier aircraft. The aerodynamic stability (above all in the pitch axis) does not improve substantially until the wings have been extended.
In the release of an unmanned missile attached to a so-called pylon on the carrier aircraft, it should be ensured that the unmanned missile is not immediately deflected after its release by aerodynamic forces, for example, by the so-called downwash field (i.e., the flow field around the missile in the attached state) and through repulsive forces of an ejector system (e.g., a gas pressure system) provided on the pylon. Such an immediate deflection can result in the unmanned missile colliding with the carrier aircraft or getting into a flight condition that can no longer be controlled.
The aerodynamic forces acting on the missile are dependent on the velocity of approach, i.e., on the tape-to-head speed of the carrier aircraft to the air upon release of the unmanned missile, on the atmospheric density, i.e., the flight altitude at which the release occurs, on the angle of incidence of the missile and on the aerodynamic flow conditions, i.e., the downwash field, of the unmanned missile attached to the carrier aircraft. This downwash field, which is different from an airflow around the missile in the free air field, results from flow bottlenecks (e.g., through containers on adjacent weapon stations or through restriction of the space between a missile and an underside of the carrier aircraft by a pitch angle offset provided during the installation of the missile) and/or from fluid-flow stagnation areas (e.g., on the front root of the pylon). In this way, static and dynamic pressure differences and thus special actions of forces and moments on the missile result, which disappear again with increasing distance from the carrier aircraft during release, typically occur approximately 200 ms after the release. Subsequently the aerodynamics of the free air flow acts on the missile.
Pressure differences between the top and underside of the missile generate pitching moments and pressure differences between the left and the right side of the missile generate yawing moments. Typically, negligible moments about the yaw axis and about the roll axis result at wing weapon stations with sufficient distance from adjacent containers. However, a dominating pitching moment results, which presses the missile nose downwards and the strength of which depends on the angle of incidence of the missile, on the Mach number and on the atmospheric density. This pitching moment already acts on the missile in the attached state and during the release exerts a more or less strong angular momentum about the pitch axis on the missile. This specific rotational effect disappears during the release operation with increasing distance of the missile from the carrier aircraft, because the pressure differences between the top and underside balance one another. Since in this situation the missile has hardly any aerodynamic pitch stability, the rotational motion about the pitch axis caused by the pitching moment continues. The pitch rotation rate greatly increases due to the increasing area of the missile, which is exposed to the dynamic pressure of the approach flow, and which results above all when the aerodynamic center of pressure is located ahead of the center of gravity (nose is at the front). Then a flight attitude of the missile very quickly occurs thereby which can no longer be stabilized, so that the released unmanned missile crashes out of control and is lost for the planned mission, unless corresponding countermeasures are initiated promptly.
Immediately after the release, the unmanned missile is subjected to a pitching moment due to the aerodynamic forces acting on it, which pitching moment presses the nose of the missile downwards that can result in the missile being in an uncontrolled vertical flight attitude. This flight attitude can no longer be stabilized even after the wings have been extended, so that the released unmanned missile crashes out of control and is lost for the planned mission.
The release of a missile from the carrier aircraft is carried out in a state of the missile in which the flight attitude control thereof has not yet been activated. This is thereby intended to avoid a collision between the missile and the carrier aircraft occurring in the immediate vicinity of the carrier aircraft through an error in the flight attitude control of the unmanned missile. For this reason, different approaches have hitherto been preferred.
Since the missile is aerodynamically stable and inactive during the carried flight, i.e., without its own power supply, it is connected to the carrier aircraft by a release cord in addition to the mechanical fastening device. The release cord activates the power supply of the missile after the release of the missile at a specific distance of the missile from the carrier aircraft, so that the flight attitude control of the missile cannot be operable until then. Only missiles with sufficiently large aerodynamic stability or with restricted release conditions with respect to angle of incidence, Mach number and altitude can be used for this method, so that a loss of the missile through the downwash field acting until the start of its own flight control is ruled out.
In an alternative approach, the power system of the aerodynamically stable missile is already active during the carried flight. However, for safety reasons, the activation of the flight attitude control of the missile is delayed via corresponding time-delay devices in the missile and the control surface deflection of the control surfaces is limited for a specific time until the missile is sufficiently far away from the carrier aircraft. During the phase of the unregulated flight between the release and the start of the flight attitude control of the missile, there is no risk of loss here either through the aerodynamic forces acting thereon because of the aerodynamic stability of the missile.
However, if the missile is aerodynamically unstable, at least in the first flight phase after the release, and if relevant forces and moments of the downwash field act on the missile, there is a risk that it will get into an unstable flight attitude in the time between the release and the start of the missile flight attitude control and will therefore be lost. However, the safety philosophy has hitherto required that the flight attitude control of the missile can be active only when it has been ensured that a faulty flight attitude control will not lead to a collision of the missile with the carrier. In practice, this period of time for aerodynamically unstable missiles has hitherto not been less than 100 ms after release from the carrier aircraft has been detected.
In the case of aerodynamically unstable missiles, the vertical rudders and elevators provided on the tail of the unmanned missile have hitherto been brought from the neutral position (rudder angle=0°) into a deflected rudder position (so-called “fin preset”) shortly before the release of the missile from the carrier aircraft. In this manner, moments result to the missile center of gravity through the approach flow of the rudder and through the lever arms of the rudders which are opposed to the moments of the downwash field acting on the missile and thus correspondingly suppress the rotary motion of the missile during release.
Since the forces and moments acting on the missile due to the downwash are essentially dependent on the flight altitude and on the airspeed of the carrier aircraft, the preset rudder angles of the elevators/vertical rudders have to be adjusted to the approach flow conditions, i.e., to the airspeed and the flight altitude. In order to determine these rudder angles, it is therefore necessary to carry out a multiplicity of flight tests and computational fluid dynamics (CFD) simulations. These flight tests and simulations have to be carried out not only for each combination of a carrier aircraft model and missile model, but in addition also for each attachment location of the unmanned missile on the carrier aircraft (for example, under the fuselage or under the wings) and for each combination of the configuration of adjacent stations for the attachment of weapons and containers. Furthermore, the presetting of the rudder angles of the missiles has to be determined anew with the introduction of new weapons or containers at adjacent stations, since the new adjacent configuration changes the downwash field of the missile and since the resulting rotary and translational effects on the missile therefore change. This shows that a huge expenditure in terms of preliminary tests and simulations has to be invested before the clearance for use of a combination of carrier aircraft and unmanned missile.